Similar to SQL Injection, XPath Injections operate on web sites that uses user-supplied information to construct an XPath query for XML data. By sending intentionally malformed information into the web site, an attacker can find out how the XML data is structured, or access data that he may not normally have access to. He may even be able to elevate his privileges on the web site if the XML data is being used for authentication (such as an XML based user file).
XPath Injections might be even more dangerous than SQL Injections since XPath lacks access control and allows querying of the complete database (XML document), whereas many SQL databases have meta tables that cannot be accessed by regular queries.